1. Field of the Invention
This invention relates to a biodegradable polymer composition that exhibits good resistance to thermal decomposition, resulting from molding or radiation sterilization. It has been found that it is possible to control the reduction in weight-average molecular weight caused by thermal decomposition to within 30% of the initial molecular weight after molding and radiation sterilization, by adding a free radical scavenger to the biodegradable polymer.
In this invention, the biodegradable polymer has sufficiently mild properties so that it is suitable for medical treatment of parts of the body and their environment, while maintaining its shape and properties for a necessary period. After such treatment, the polymer may be caused to disappear by hydrolyzing with an enzyme or non-enzyme.
2. Description of the Related Art
The biodegradable polymer may be natural or synthetic, and an enzyme is capable of hydrolyzing almost all natural polymers. For example, collagen which is a polypeptide, is representative of the natural polymers and is hydrolyzed by collagenase, while a polyglycoside composed of combined glycosides such as cellulose, starch, hyaluromic acid, chitin and chitosan is also contemplated, with cellulose, for example being hydrolyzed by cellulase enzyme.
In spite of the fact that a natural polyester produced by a microbe was known as a biodegradable polymer since 1920, such knowledge was not utilized for a long time. However, as a result of recent progress in biotechnology, many kinds of natural polyesters have been researched and developed as biodegradable materials, including poly β-hydroxybutylate.
Many kinds of synthetic biodegradable polymers are capable of being hydrolyzed by a non-enzyme. However, polypeptides such as poly glutamic acid are hydrolyzed by peptide decomposition enzymes similar to natural polypeptides.
Almost all biodegradable synthetic polyesters such as polyglucolic acid, polylactic acid or copolymers of glycolic and lactic acids, are hydrolyzed by non-enzymes and find frequent clinical use as medical materials.
There are many medical applications of biodegradable polymers employed as medical materials, but these are almost always restricted to surgical use such as suture or bone fixation materials. Other industrial uses of these polymers include PLLA film molded products such as a garbage bags, agricultural film, storage bags, or textiles, since PLLA is decomposed by microbes under a natural environment.
The production of biodegradable polymers useful as medical and industrial materials is accomplished by such methods as extrusion of heated meltdown, injection, and pressed molding. However, heating processes are usually avoided in the course of production, since a drop in molecular weight inevitably occurs in products after such heating process, because biodegradable polymers generally have poor heat stability. In addition, sterilization is inevitably necessary for medical use in contrast with industrial use, because biodegradable polymers are applied in surgical use such as surgical sutures or bone fixation materials. Ethylene-oxide gas is generally utilized for the sterilization process, because biodegradable polymers show poor durability against radiation exposure. In this process, since ethylene-oxide gas utilized for the sterilization, is intended to eliminate toxicity in the living body, it is inevitably necessary to remove residual gas after the sterilization process, by applying vacuum for an extended period, but it is nevertheless impossible to completely remove the gas. Sterilization methods by radiation are therefore employed in many cases when biodegradable polymers are utilized for medical uses. Moreover, only specified kinds of biodegradable polymers are generally irradiated, but some strength deterioration inevitably occurs, caused by radiation decomposition.
It is the object of this invention to treat biodegradable polymers so as to avoid or restrict molecular weight reduction in the course of heat treatment processes, and strength deterioration, caused by sterilization, and to improve the properties of the composition containing the treated polymer.